Research brief: Influence of the Madden-Julian oscillation on Costa Rican mid-summer drought timing

The Central American mid‐summer drought (MSD) is the decline of precipitation during the middle of the wet season (July and August) over Central America and southern Mexico, which has been shown to have strong effects on agriculture and the initiation of bushfires in Costa Rica. The Madden–Julian oscillation (MJO) has been shown to influence Costa Rican rainfall on intra‐seasonal time scales, and therefore may be important to the MSD. Therefore, this research aims to find the connection between the MJO and MSD.

We used rainfall data from seven stations to simulate the precipitation distribution in Costa Rica. We found that MSD signals are evident in four of the seven stations located in the Pacific coast of Costa Rica. Annual MSD signals were determined in these four stations, characterised by onset, end and intensity. Finally, we counted the number of onset and end dates allocated in each MJO phase for each station and used a statistical method to show that MSD tended to start/end in MJO phase 1 and 8. This phenomenon was then dynamically explained by the wind circulation anomalies over Costa Rica, showing that phase 1 and 8 tended to bring eastward wind anomalies to the Pacific coast and induce the large rainfall corresponding to onset/end of the MSD. This research highlights the potential of the MJO’s application in MSD prediction, which could be beneficial for agriculture in Central America.